The present invention relates to a method for purging a fuel cell for the purposes of reducing the water content in the fuel cell, and to a device for carrying out the method.
For a successful frost start of a fuel cell system, it is necessary to reduce the amount of water contained in a fuel cell or in a fuel cell stack of the fuel cell system, in order to prevent freezing of the water therein and in order to lower a thermal capacity of the fuel cell system, For this purpose, in the prior art, the fuel cell system is blown dry upon being shut down. The moisture content may, for example, be reduced in that, after the deactivation of the fuel cell or of the fuel cell system, the operation of the compressor is continued for a certain period of time in order to extract moisture from the fuel cell. This approach is also referred to as follow-on running or “blow down”. Owing to the pressure conditions prevailing in the fuel cell system, this method is however inefficient, and adequate drying of the system is scarcely possible.
DE 60 2004 000440 T2 discloses a fuel cell drive unit having a motor-compressor assembly for supplying compressed air to the fuel cell, wherein the direction of rotation of the motor-compressor assembly can be reversed.
Proceeding from the prior art, it is an object of the present invention to provide a method for purging a fuel cell, by way of which method the amount of water in the fuel cell can be reduced in a simple and efficient manner. It is also an object of the present invention to provide a device for carrying out the method, which device is also suitable for a frost start or allows for a shut-down strategy in preparation for the frost start. It is also a preferred object of the technology disclosed herein to prepare the fuel cells for the next cold start or frost start, without harmful gases or contaminants, for example introduced particles, passing into the fuel cell in the process. Moreover, it is an aspect of the technology disclosed here to increase safety during the follow-on running and to reduce any emissions (noise, water).
These and other objects are achieved by a method for purging a fuel cell, which method has the step of suctioning purging gas through the fuel cell by way of negative pressure. By way of the suctioning of purging gas, the absolute pressure acting in the fuel cell is reduced in relation to a situation in Which purging gas is blown through the fuel cell. In this way, the liquid water contained in the fuel cell is evaporated at a lower temperature, and drying is thus achieved very quickly and easily. The purging gas is not restricted in terms of its detail. Purging gas is generally the gas which is purged through the fuel cell in order to reduce the amount of water after the “blow down” operation of the fuel cell, without this serving the purposes of the electrochemical reaction in the fuel cell. In the present case, the purging is performed by suctioning. It is, however, preferable for fresh air to be used as purging gas, because material costs for the purging gas are thus omitted, and fresh air is freely accessible almost everywhere. The method according to the invention is extremely highly suited to fuel cell systems with one fuel cell or with a fuel cell stack, wherein the method can be performed inexpensively, without a high level of technical outlay, and in a short time.
In one advantageous refinement of the method according to the invention, in order to generate the negative pressure, a delivery device which supplies cathode gas to the fuel cell is reversed in terms of its delivery direction. This may be realized most easily by electronic means. Cathode gas is in this case generally the gas which is supplied to the fuel cell during the normal or standard operation of the fuel cell (that is to say not during the follow-on running phase) for the electrochemical fuel cell reaction.
It is advantageous for the entire air path of the fuel cell to be briefly reversed in terms of its delivery direction. Purging gas, which includes fresh air, may in this case be suctioned in from surroundings of the fuel cell and supplied for example via an inlet air filter. The amount of fresh air to be delivered, and the pressure level thereof, may preferably be controlled by way of a throttle flap. The delivery device is preferably a compressor operated in the reverse direction, which compressor supplies cathode gas to the fuel cell during normal use. This embodiment of the method requires substantially no structural changes to the fuel cell system, and is therefore preferable in particular with regard to the costs for carrying out the method.
A further advantageous refinement provides that, for the generation of the negative pressure, a delivery direction in the fuel cell is reversed. This relates in particular to the cathode path of the fuel cell. In this way, the fuel cell can be purged in a highly effective manner.
It is furthermore advantageous for purging gas to be suctioned in via a fuel-cell exhaust-gas line. Since the fuel-cell exhaust-gas line is connected directly to the fuel cell to be purged, no separate purging-gas supply is required in order to deliver, in particular, fresh air from the fuel cell surroundings. The suctioning-in may in this case take place directly from the fuel-cell exhaust-gas outlet or via a supply line which opens into the fuel-cell exhaust-gas line.
From the aspect of a simple method implementation, the purging gas is preferably suctioned in via a fuel-cell exhaust-gas outlet. During the normal operation of the fuel cell, the fuel-cell exhaust-gas outlet conveys anode exhaust gas, cathode exhaust gas or fuel cell mixed exhaust gas, which may also contain water, into the surroundings. It may thus duly be the case that, initially, water-containing exhaust gas is suctioned in through the fuel cell by negative pressure, but mixing with suctioned-in fresh air is possible already in the fuel-cell exhaust-gas line, such that fast and simple drying of the fuel cell is achieved. To reduce or prevent water-containing fuel cell exhaust air from being suctioned in, it is particularly preferably possible for the normal delivery direction to initially be maintained for a few seconds, preferably for 1 to 10 seconds, in particular for 2 to 5 seconds, after the shutdown of the fuel cell before purging gas is suctioned in via the fuel-cell exhaust-gas outlet.
Alternatively or in addition thereto, purging gas may be suctioned in via a purging-gas supply that is additionally provided in a cathode-gas supply of the fuel cell. An advantage of this is that fresh air can be suctioned through the fuel cell by way of negative pressure immediately, without water-containing fuel cell exhaust air having to be conveyed. The drying time of the fuel cell is reduced in this way. For this embodiment of the method according to the invention, the delivery device is operated in the normal direction.
It is furthermore advantageously provided that the purging gas that is suctioned through the fuel cell is delivered out via a cathode-gas supply. This promotes a fast expulsion of now humidified purging gas from the fuel cell system.
For further simplification of the method, the purging gas that is suctioned through the fuel cell is delivered out via a purging-gas outlet which is provided in a cathode-gas supply of the fuel cell.
For further shortening of the drying time, water is condensed out of the fuel cell exhaust air and/or out of the purging gas before the suctioning through the fuel cell.
Likewise specified according to the invention is a device for carrying out the method described above. The device is, for example, a fuel cell system and includes at least one fuel cell having a cathode-gas supply for the supply of cathode gas to the fuel cell and having a fuel-cell exhaust-gas line for the delivery of anode exhaust gas, cathode exhaust gas or fuel cell mixed exhaust gas out of the fuel cell. In the cathode gas supply there is provided a delivery device, in particular a compressor, which is designed to generate negative pressure in the fuel cell and to suction purging gas through the fuel cell. By way of the embodiment of the device according to the invention, which permits suctioning of purging gas rather than forcing of purging gas through the fuel cell by way of high pressure, drying of the fuel cell is possible within an extremely short time, and a fuel cell with good frost start characteristics can be provided.
The advantages, advantageous effects and refinements presented for the method according to the invention also apply to the device according to the invention for carrying out the method.
With regard to a simple component arrangement, the delivery device is a cathode-gas delivery device for supplying cathode gas to the fuel cell, which cathode-gas delivery device is operated in a reverse delivery direction. This is easy and inexpensive to implement by way of electronic control. It is furthermore possible to dispense with additional delivery devices and to provide a device which has a minimum number of functional components.
A further advantageous refinement provides that the device furthermore includes a purging-gas supply and/or a purging-gas outlet, which are arranged in the cathode-gas supply between the delivery device and the fuel cell. It is thus possible for the delivery device to be operated in a normal delivery direction, and for purging gas to nevertheless be suctioned through the fuel cell with high efficiency. The operation of the delivery device in the normal delivery direction is particularly advantageous with regard to the geometric arrangement of the delivery device components configured in the delivery direction, normally blade geometry, and a thus increased delivery rate.
Further advantageously, the device according to the invention includes a first bypass line and/or a second bypass line, wherein the first bypass line connects the fuel-cell exhaust-gas line and the cathode-gas supply downstream of the delivery device, and wherein the second bypass line connects the cathode-gas supply downstream of the delivery device to the cathode-gas supply upstream of the delivery device. In this way, from a structural aspect, simple delivery paths are provided for the purging gas to be suctioned through the fuel cell, without major intervention into the architecture of the device.
For simplification of the control of the flow direction of the purging gas, the first bypass line and the second bypass line are connected to one another by way of a multi-way valve.
To reduce the time required for the drying of the fuel cell, in particular in the case of fresh air suctioned in through the fuel-cell exhaust-gas line being used as purging gas, the device includes at least one condenser arranged in the fuel-cell exhaust-gas line. A condenser or water separator may alternatively or additionally also be provided at the fuel cell-outlet upstream of the branching point of the first bypass line.
The purging gas that is suctioned through the fuel cell can be delivered out via a fuel-cell exhaust-gas outlet. For example, the purging gas suctioned through the fuel cell can be delivered into the fuel-cell exhaust-gas line via a first bypass line or a fuel cell bypass (below, for simplicity, the expression “first bypass line” will be used). This has the advantage that components provided in the fuel-cell exhaust-gas line, such as for example the exhaust-gas system and/or the water separator, which are provided in any case for the normal or standard fuel cell operation, can also treat the purging gas correspondingly. Thus, a situation is preferably prevented in which the moisture purged out of the fuel cell passes into the surroundings. Furthermore, the noise level can be reduced by way of the noise-reduction measures of the exhaust-gas system. Moreover, evaluation devices positioned downstream of the fuel cell, such as for example the device for determining the hydrogen concentration in the fuel-cell exhaust gas, can also be used for the purging gas. If, during the follow-on running, the purging gas is discharged through the fuel-cell exhaust-gas outlet that is utilized for standard operation, the hydrogen concentration in the fuel-cell exhaust-gas line is simultaneously reduced. It would, however, alternatively or additionally be possible for a purging-gas outlet, through which the gas escapes into the surroundings, to also be provided upstream of the delivery device.
In particular, the purging gas may also be suctioned in via the cathode-gas supply of the fuel cell. The purging gas may initially be filtered by the cathode-gas air filter of the fuel cell before being suctioned through the fuel cell. The cathode-gas air filter is the filter which, during the standard operation of the fuel cell, removes any contaminants and/or harmful gases from the gas that participates in the reaction on the cathode side of the fuel cell. Alternatively or in addition, it is contemplated for the purging gas to be filtered by a separate filter. The use of the cathode-gas air filter is, however, particularly preferable because, in this way, structural space, production costs and servicing costs can be reduced.
During the suctioning of purging gas upstream of the fuel cell, the purging gas may be suctioned in via the cathode-gas air filter in such a way that flow does not pass through the delivery device. During the suctioning of purging gas, the delivery device may be connected via at least one line to the fuel-cell exhaust-gas line, in such a way that the delivery device suctions in purging gas through the fuel cell. In the line downstream of the delivery device and upstream of the fuel cell, there is already generated a negative pressure which makes it possible for moisture to be condensed out. It is preferable for a water separator to be provided in the line between the fuel cell and the delivery device, in order that as little moisture as possible passes into the delivery device. The water separator is particularly preferably provided between the fuel cell and (first) multi-way valve that is arranged in the fuel-cell exhaust-gas line. It is thus possible for one water separator to be used for normal operation and for follow-on running.
It is possible for multiple multi-way valves to be provided, which may be arranged in the cathode-gas supply and/or in the fuel-cell exhaust-gas line, and designed, such that, during normal operation of the fuel cell, they conduct cathode gas to the fuel cell and discharge fuel-cell exhaust gas out of the fuel cell through the fuel-cell exhaust-gas line and/or through the fuel-cell exhaust-gas outlet. Furthermore, the multi-way valves may be arranged and designed such that, during the suctioning of purging gas, they conduct purging gas from the cathode air filter into the fuel cell and supply the purging gas, downstream of the fuel cell, to the delivery device, such that the delivery device suctions in filtered purging gas through the fuel cell.
Also described is the use of a delivery device, operated in a reversed delivery direction, for generating negative pressure in a fuel cell for the purposes of purging the fuel cell with purging gas.
The solutions according to the invention, and the refinements thereof, yield the following advantages:                An easily implementable and inexpensive method for the purging and drying of a fuel cell is provided.        The device including at least one fuel cell is highly suitable for a frost start.        The device can be provided with minimal structural modifications.        A simplification of the method implementation is possible merely by modification of the electronic control, even without structural modification of the device.        The existing components can be used for the treatment of the purging gas upstream and downstream of the fuel cell. Space requirement, production costs and servicing costs are reduced.        The existing safety systems can monitor the purging process.        
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.